1. Field Of The Invention
The invention relates to drill pipe for subterranean wells, and the like, having welds to connect pipe sections to each other.
2. Brief Description Of The Prior Art
When drill pipe is used to drill subterranean wells, the pipe is exposed to bending stresses. Such bending stresses are primarily due to hole curvatures extending through the entire length of the drilled hole. Such stresses may cause fatigue of the drill pipe due to fluctuating reversed bending stress which is imposed during rotation of the drill pipe. During these fluctuating reversed bending stresses, this portion of the pipe is alternately subjected to pressure, tensile, and twisting or torsional forces as the drill pipe rotates. If the drill pipe fails by such resulting fatigue, the location of the fatigue point is oftentimes approximate the area of securement thereof, i.e., from about 1 to about 5 feet from the point of securement of the top end of the pipe to the tool joint. The stress in this portion of the drill pipe is usually considerably higher than the stress that is imposed on the remaining portions of the drill pipe conduit.
If pipe fails by fatigue, the fatigue often originates in a slip mark. Slip marks occur at the end of the pipe attached to the tool joint, because rotatory slips are used to support the drill string during the make and break cycles while drilling and tripping out of a well hole occur. These slips act as wedges that hold the entire weight of the drill string. The portion of the slips that touch the pipe have teeth that can dig into or notch the drill pipe. These notches can act as stress risers that can act as a site for premature fatigue crack initiation and propagation. The above-mentioned bending stresses can initiate these fatigue cracks, thus causing the pipe to fail.
A portion of drill pipe consisting of a thicker, more fatigue-resistant material may be used to resist these higher stresses to diminish the tendency of the pipe to fail by fatigue in the area of securement by slips more often than in other areas of the pipe. The thickened fatigue-resistant section has a wall thickness thicker than the wall thickness of the non-thickened remainder of the drill pipe. The thicker wall thickness of the fatigue-resistant pipe section may be created either by making the outer diameter of the fatigue-resistant section greater, or the inner diameter smaller, or some combination of the two.
The fatigue-resistant section of drill pipe may be secured by flash, friction electron beam, pressure welding, or any other acceptable technique at its lower end to the non-thickened remainder of the drill pipe. The adjoining or top end of the non-thickened remainder must be upset or gradually flared out in wall thickness so that the wall thicknesses of the ends of the thickened and non-thickened sections, as well as the sections' outer and inner diameters, are equal at the ends that are to be secured together.
In the prior art, the welded ends of the thickened and non-thickened sections of the drill pipe string sections were cut at an angle perpendicular to the pipe sections, such that the cross section of the ends are circular. When the pipe string sections are welded together using such circular welds, the aforementioned fluctuating reversed bending stresses tend to impose high stresses on the circular weld. Additionally, heavy loads imposed on the pipe string as well as shock loading put high stress on the weld. Thus, even where a thickened, fatigue-resistant section is incorporated into the pipe string in order to be more resistant to failure due to fatigue caused by slips, failure may still occur at the weld due to these stresses. Obviously, if the circular weld fails, the increased resistance to failure of the thickened section is not as beneficial as it would be were the weld more hardy.
Applicant is aware of the following prior art which is addressed to similar problems of stress on subterranean well conduits, but which does not anticipate or render obvious the present invention: (1) U.S. Pat. No. 4,416,476, issued Nov. 22, 1983, and entitled "Intermediate Weight Drill Stem Member"; (2) U.S. Pat. No. 1,325,073, issued Dec. 16, 1919, and entitled "Method Of Forming Coupling Ends On Drill Tube Sections"; and (3) U.S. Pat. No. 3,080,179, issued Mar. 5, 1963, and entitled "Slip Engaging Portion of Drill String Formed of Increased Wall Thickness and Reduced Hardness."
The present invention addresses some of the deficiencies of the prior art's use of circular welds to secure the thickened, fatigue-resistant section of pipe to the remainder of the pipe, by the use of a helical weld having increased cross section and thus improved resistance to stress and also due to its helical shape.